In bacteria, proteolysis of glutamine sythetase is preceded by an oxidative inactivation of the enzyme. This oxidative inactivation is mimicked by a nonenzymic system consisting of ascorbic acid, iron, and oxygen. Ascorbate-inactivated glutamine synthetase was studied to identify the altered amino acids. Only a single alteration was detected: one of 16 histidine residues was lost. The time course of loss of the residue matches the time course of inactivation. The ascorbate system may proceed via reduction of ferric to ferrous iron. Ferrous iron could then bind to glutamine synthetase and generate an oxidizing species which destroys the histidine residue. Such oxidative modifications may provide another mechanism for regulation of enzymic activity, perhaps reversibly. In the case of glutamine synthetase, oxidative inactivation may "mark" the protein for subsequent proteolytic degradation. Oxidative modification may also function in neutrophil-mediated host defense against bacteria.